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DAC161P997: Isolated design

SM
Expert 1750 points
Part Number: DAC161P997

Hello,

       We are building a 4-20mA transmitter using the DAC161P997. It is for industrial applications so an isolated design is preferred. We would like your advise about the following:

1) The mcu controlling the DAC is powered from a user supplied 24V (24V to 3.3V non isolated dc dc converter)

2) An isolator bridges the mcu and the DAC.

3) The DAC is powered from an isolated supply (24V to 3.3V to isolator to isolated 3.3V)

The 4-20mA loop also needs a supply which will be provided from the 4-20mA receiver which is nothing but the user supplied non isolated power. Please see the following figure. Doesn't this defeat the purpose of isolation.

Please advise on the what is missing in our understanding and what is the right way to do it.

Looking forward to your help,

  • 0
    TI__Guru 53703 points

    So to clarify,

    You are building a 3 wire transmitter? In that, it will have the 4-20mA loop as well as a third power supply connection for the MCU side of the circuit?

  • 0 in reply to Paul_Frost
    Expert 1750 points

    We are looking to build a two wire transmitter and are wondering how the loop would get closed.

  • 0 in reply to SM
    TI__Guru 53703 points

    Hi SM,

    If you are trying to do a 2 wire transmitter only, then the isolation in the figure does not really work.  The DCDC converter in the image would be backwards in that image.  The loop would be the supply for the switchmode power supply, and that would supply the isolated MCU in the circuit.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Expert 1750 points

    Hello Paul,

                Thank you for the input. May I ask you to please elaborate as we are quite confused now.

    There is some other circuitry accompanying the UC so I am not sure how we would feed all of them from the loop. In case of the current loop does it make sense then to have an isolated interface at all?

    Thank you for your guidance.

  • 0 in reply to SM
    TI__Guru 53703 points

    Hi SM,

    I guess you need to consider what your application is and what you are really trying to isolate.  If you have a critical sensor that you want to protect from the loop, then you might just want to isolate that.  If you are trying to protect the loop from your sensor and support circuitry, then you should put the isolation barrier between the sensor/MCU and the DAC.  

    What is your main concern in which you would consider isolation?  If your application cannot run on about 3.5mA, then I think you need a third wire for an additional supply, or you need to have some local power source.

  • 0 in reply to Paul_Frost
    Expert 1750 points

    Hello Paul,

                 Specifically for the application on hand, we will be feeding a signal to a PLC. Please advise on what will be the right way forward.

    Thanks for your inputs again

  • 0 in reply to SM
    TI__Guru 53703 points

    So I guess you need to decide if any isolation is necessary.  Isolation is only useful for device safety or noise performance.  If that is not critical (in that you do not expect damage to the device or that you are not in a noise critical application), then you do not need isolation.  In that case the barrier is not necessary.

    Your next step would be to estimate the total current required by your application.

    Thanks,

    Paul

  • 0 in reply to Paul_Frost
    Expert 1750 points

    Hello Paul,

                Wouldn't it be easier if the loop was not powering our transmitter? How would the design be in that case? 

    The mcu and DAC can be powered from the same 3.3V but what about the loop part. If the DAC is sourcing the current then measuring the potential drop across a resistor (and not the between the resistor and ground) will do the job. There will not be any need to sink current at the receiver side.

  • 0 in reply to SM
    TI__Guru 53703 points

    Hi SM,

    Here is an article my colleague wrote about 3-wire design with isolation, I think it will give you a good understanding about the system.  In this case the DAC161P997 will replace the DAC87x0.

    https://e2e.ti.com/blogs_/archives/b/precisionhub/archive/2015/06/15/input-isolation-for-high-power-sensor-transmitters

    Thanks,

    Paul